In this state of things the counter weight of the great piston rod will, by its preponderance, descend in its barrel, and cause the piston B to rise in the cy linder. When the piston B has risen half way up, the equilibrium valve K closes ; and when the piston is quite at the top, the exhaustion valve L, the injection valve N, and the expansion valve G, arc all opened by the action of the catches upon the handles of the work ing gear.
The steam consequently again rushes through G to the top of the piston, while the steam beneath it es capes into the condenser, where it receives the jet of the injection, and is converted into water, again cre ating a vacuum beneath the piston B, and in the con denser. The piston B consequently again descends, and the engine continues to act in the same manner, and thus to work the pump h j.
The air pump P does not begin its operation till the water of condensation and the injection water have accumulated. When this is the case, this water, flow ing through II, rises above the piston Q through its valve ; whenever it is at the bottom of the cylinder P, and the valve closed upon it, and preventing its de scent, it is raised and discharged through the valve S into the hot well. In like manner, if there is any air or vapour in the condenser, it will help to press the water through II, and will itself escape through the valve in the box Q, when all the water has been dis charged at S.
The pump UU, which enters the hot well, forces the water from it along the tube u and feeding pipe t, to supply the boiler n, so that there must be a saving of fuel by using this hot water in place of cold water.
Although we have mentioned generally the con struction of the plug tree, and the way in which it opens and shuts the valves, yet as this is an essential part of the engine, we shall give a separate drawing of the mechanism . which opens and shuts the nozzle valves and regulator, along with Mr. Watt's own de scription of it. "The piston approaching the top of the cylinder, the slider a fastened upon the plug tree z, raises up the handle b, which is fixed upon the lower V shaft or axis c, as is the detent d, and the latter takes hold of the double ended catch e ; but, in doing this, the upper end of the catch allows the de tent f to escape, and a weight hung to the rod g turns the axis it. The arm i and rod j are moved out of the
straight line at 1, and by a lever k turn the spindle m in the upper nozzle, which, by means of a toothed sector n, and rack o, raises the valve p, and admits steam into the cylinder above the piston through the horizontal pipe A. At the same time, another arm u, fixed upon the same shaft, by means of the rod w, acts upon a spindle, &c. in the lower nozzle, and opens the exhaustion valve L (Plate DVII. Fig. 1.), and thereby forms a communication between the cy linder below the piston and the condenser. The pis ton now descending, another slider g moves the han dle r into the position s ; this raises the weight g, while i and k are brought back to the position 1, and the valves p and L are shut. The detent f, in acting upon the catch e, disengages d ; the lower V shaft turns upon its axis, and two arms attached to it (sim ilar to those upon the upper Y shaft, which arc omit ted to avoid confusion) by means of the rod x and y, open the lower steam valve K (Plate DVII. Fig. I.) and the upper exhaustion valve t. The cylinder above the piston becomes exhausted, and the steam, enter ing below it, causes the piston to re-ascend." The steam engine of the form, which we have now described, continued long in use for draining mines. Between 1778 and 1790, great numbers of them were erected by Messrs. Bolton and Watt, in different parts of England, but chiefly in Cornwall. Their cylin ders were generally from forty-eight to sixty-six inches in diameter, and according to Mr. Watt, they were found to raise from twenty-four to thirty-two millions of pounds of water one foot high by means of one' bushel of good Newcastle or Swansea coals, when working more or less expansively.
As the action of these engines is suspended during the ascent of the piston, they were particularly suited for pumping up water, since during the intermission the piston of the pump descended to make a new stroke. But when a continued power was required for the purpose of driving machinery in which there was no intermission of action, this form of the steam-en gine was by no means applicable.